Aluminum alloy heat-treating

ABSTRACT

A METHOD FOR HEAT-TREATING ALUMINUM ALLOYS WHEREIN THE COMPLETELY SOLIDIFIED MATERIAL IS SOLUTION AND HOMOGENIZATION HEAT-TREATED IN A VACUUM ENVIRONMENT AT A TEMPERATURE IN THE RANGE EXTENDING FROM THE ALLOY SOLIDUS TEMPERATURE TO THE ALLOY SOLID SOLUBILITY LIMIT TEMPERATURE A SUFFICIENT TIME TO DEVELOP IMPROVED PHYSICAL AND METALLURGICAL PROPERTIES UPON SUBSEQUENT COOLING AND AGING.

United States Patent 3,598,663 ALUMINUM ALLOY HEAT-TREATING HerbertGreenewald, Jr., Columbus, Ohio, assignor to North American RockwellCorporation No Drawing. Original application Mar. 13, 1968, Ser. No.712,674, now Patent No. 3,515,546, dated June 2, 1970. Divided and thisapplication Mar. 25, 1968, Ser. No.

Int. Cl. C21d 1/74 US. Cl. 148-159 4 Claims ABSTRACT OF THE DISCLOSURECROSS-REFERENCE This application is a division of application for US.Pat. Ser. No. 712,674, filed Mar. 13, 1968 for Aluminum Alloysolidification, issued June 2, 1970 as US. Pat. No. 3,515,546, andassigned to the assignee of this application.

SUMMARY OF THE INVENTION Aluminum alloys having a significant zinc,magnesium, or other strong hydride former content, and particularly ifmelted, poured, or solidified in an environment containing excessivehydrogen, hydrocarbon, or water vapor constituents, are solution andhomogenization heat-treated in accordance with this invention in avacuum environment at a temperature in the range extending from thealloy solidus temperature to the alloy solid solubility limittemperature a sufficient time. For 7075-type aluminum alloys modifiedfor casting applications, for instance, heattreatment in a vacuum of 210 mm. Hg at a temperature of 860 F.:40 F. for 24 hours per 0.1 ofsolidified alloy thickness aids substantially in obtaining improvedphysical and metallurigical properties. Afterwards, the solutioned andhomogenized alloy is preferably immediately quenched and subjected topost-heat-treat aging such as on an accelerated basis by heating at from250 F. to 450 F. for from 24 hours to 2 hours.

DETAILED DESCRIPTION The heat-treating method of this invention hasapplication to the type of aluminum alloy detailed as to composition inthe following Table I:

Such compositions are stated on a percentage part by weight basis andare characterized as having a significant 3,598,663 Patented Aug. 10,1971 zinc, magnesium, or other strong hydride former content.

The instant invention does not involve melting, pouring, or completelysolidifying alloy compositions and such may be accomplishedconventionally or even in accordance with the pre-heat treatment methoddisclosed and claimed in the above-cross-reference application forUnited States patent, for instance, depending on applicationrequirements. However, significant advantages may be attained if thecompletely solidfied alloy is solution and homogenization heat-treatedaccording to the following method, particuarly if allo melting, pouring,or solidification prior to heat-treatment has been accomplished in anenvironment having excess hydrogen, hydrocarbons, or water vapor.

The first step in the preferred solution and homogenizationheat-treating method involves placing the solidified aluminum alloyhaving a significant zinc, magnesium, or other strong hydride formercontent in a furnace chamber that may be sealed and pumped down to acontinuous vacuum condition. Solution and homogenization is thenaccomplished after the chamber pressure is reduced to a vacuum such as2x10" mm. Hg. (absolute). The preferred temperature for suchheat-treating will vary in relation to the alloy composition. In thecase of the modified 7075-type material, 860 F.i40 is generallypreferred. Alloy 220 can be heat-treated at a temperature in the rangeof 800 F. -4O F.; 950 F. i40 F. is considered satisfactory forheat-treating the other alloy detailed as to composition in Table 1.Such heat-treating temperatures, in each instance, are in thetemperature range from the alloy solidus temperature to the alloy solidsolubility limit temperature. For the alloys specified in Table I, 24hours at temperature per 0.1" of metal thickness is generallysatisfactory from the standpoint of time.

Post-heat-treat cooling follows the vacuum solution and homogenizationheat-treating step and preferably occurs at cooling rates that aresufiiciently high to obtain a non-equilibrium metallurgical structure.It is therefore preferred that the alloys of Table I be immediatelyquenched from the solution and homogenization heattreating temperatureto ambient temperature in ambient temperature water, equipmentpermitting. This differs from the normal practice of accomplishingpost-heat-treat cooling in heated water such as 180 F., for instance andwith an intermediate heating following cooling in air.

Afterwards, the quenched alloy may be aged naturally or by conventionalaccelerated aging steps. In terms of the aluminum alloys of Table I,accelerated aging temperature-time histories in the range of from 450 F.for 2 hours to 250 F. for 24 hours are adequate. No environmentalcontrolled atmosphere or environmental controlled pressure is required.1

A casting alloy having a composition of the type corresponding to themodified 7075-type aluminum alloy set forth in Table I was melted,poured, and directionally solidified in accordance with thepre-heat-treatment method set forth in the description and drawings ofthe cross-reference application for United States patent. The moldcavity incorporated the external configuration of a missile fin andincluded a joining riser region positioned above the fin configurationregion. A portion .of the completed casting was solution andhomogenization heattreated in a 2X 10- mm. Hg continuously pumped vacuumenvironment at a temperature of approximately 860 F. for 48 hours. Sincethe available heat-treating equipment did not permit quenching directlyfrom temperature, the part was air-cooled after solution andhomogenization heat-treating and afterwards heated to 860 F. in an airfurnace for 2 hours and then water-quenched as specified. Round tensiletest bars were machined from coupons cut from the casting and were foundto have the properties set forth in the following Table II.

Another portion of the same casting was subjected to a differentheat-treatment and then subjected to testing. specifically, thesolidified 7075-type alloy was solution and homogenization heat-treatedconventionally in an argon atmosphere just above standard atmosphericpressure at a temperature of 860 F. for 72 hours. The casting portionwas air-cooled, reheated to 860 F. for 2 hours in air, water-quenched,and aged as specified (i.e., 24 hours at 250 F.). A coupon from thecasting portion so heat-treated, cooled, and aged was found to have ayield tensile strength of 55,300 p.s.i., an ultimate tensile strength of56,100 p.s.i., and percent elongation of 1.2%.

The completed and heat-treated casting portions were examinedradiographically and found to be free of observable defects,particularly as to pin-hole porosity. The improved physical andmetallurgical properties first given above are attributed to theinvention claimed herein. Vacuum pressure levels are selected withreference to the apparent volatility of alloy constituents at the metaltemperature involved and so as to preclude alloying constituentdepletion due to processing requirements. More specifically, a vacuumenvironment of 1 10- mm. Hg has been determined to be unsatisfactory inat least one instance wherein the aluminum alloy zinc content wasundesirably reduced from 5.5% to 0.2%.

Alsoby way of example, a casting having an aircraft elevator fittingconfiguration was cast in an alloy whose melt composition (average)consisted of 1.47% copper, 5.62% zinc,-2.6l% magnesium, 0.2% titanium,0.16% beryllium, O.l% manganese, O.l% chromium, 0.1% iron, 0.08%silicon, and the balance aluminum, using the pre-heat-treatment melting,pouring, and directional solidification steps disclosed in thecross-reference application for United States patent. After conventionalheat-treating in an argon atmosphere at 860 F. for 72 hours followed byquenching and conventional accelerated aging. Coupons cut from theso-processed casting were found to have ultimate tensile strengths from58,900 p.s.i. to 70,600 p.s.i., elongations from 1.4% to 4.1%, andhardnesses (average) from R 78 to R 89. One other coupon from thecasting was subjected to solution and homogenization heat-treating asspecified above in the description of this invention for an additional23 hours at 860 F. using a vacuum of 2 10- mm. Hg. After water-quenchingand aging at 250 F. for 24 hours, the coupon was found to have anultimate tensile strength of 92,200 p.s.i., a 3% elongation, and anaverage hardness of R 82.5.

What is claimed is:

1. A method of heat treating a precipitation hardenable aluminum alloyto develop improved physical and metallurgical properties, comprisingthe steps of:

(a) heating said alloy in a non-heat-treated solidified condition at atemperature in the temperature range from the alloy solidus temperatureto the alloy solid solubility limit temperature and in a vacuumenvironment at a pressure above the pressure of alloying elementdepletion by vaporization at temperature a sufiicient time to dissolvesecond phase precipitates contained in said alloy and to compositionallyhomogenize said alloy.

(b) cooling said heated alloy at a rate sufficient to obtain anon-equilibrium metallurgical structure, and

(c) precipitation hardening said heated and cooled alloy by aging todevelop improved physical and metallurgical properties therein.

2. The invention defined by claim 1, wherein said heating is continuedfor approximately 24 hours per 0.1" of alloy thickness.

3. The invention defined by claim 1, wherein said vacuum environment isat a pressure of approximately 2 10- mm. Hg absolute.

4. The invention defined by claim 3, wherein said heating is continuedfor approximately 24 hours per 0.1 of alloy thickness.

References Cited UNITED STATES PATENTS 1,996,379 4/1935 Keller 148162,221,526 11/1940 Sampson 148l59 2,995,479 8/1961 Cochran et al. 148l3.13,084,080 4/1963 Hunter et al. 148-13.1X

OTHER REFERENCES Chemical Abstracts, vol. 67, 1967, Abstract No.46505(e).

CHARLES N. LOVELL, Primary Examiner

